How to spin more than one propellers(objects) at the same time
Using Drivers.

Similarly to my answer here I believe drivers are a great way to animate mechanical objects.

For a very simple case the rotation axis, in this case z, can be "spun" by setting the rotation to the frame, by typing frame (known to drivers, as the current frame) into the field.





Adding other variables as custom properties on the parent empty, eg on / off / speed, as well as on the individual rotor empties themselves, could change the speed of each.